Fundamental goals of research and development in the field of optical data communication are energy-efficiency and error-free data transmission across maximum distances at largest bit rates. For conventional data transmission systems based on multiple-mode optical fiber lengths of less than 2 km, as for supercomputers or between servers of data centers, high modulation bandwidth VCSELs are indispensable. In order to optimize the VCSEL bandwidth, the differential gain (∂g/∂N) is maximized ∂g/∂N is maximized, the active volume Vg is minimized (for example by reducing the optical cavity length to λ/2), and large pump currents are achieved by minimizing the resistance and the thermal conductivity [1, 2]. An additional degree-of-freedom, that has been largely overlooked until now, is the optimization of the cavity photon lifetime, essentially controlled by mirror reflectivity. Here we describe a low cost method to adjust the mirror reflectivity to optimize the cavity photon lifetime.
Previous approaches to adjust the cavity photon lifetime used dry etching of the top mirror surface [3, 4]. Very low etching rates and very shallow depths as well as an extremely precise control of the etching progress and the homogeneity across the surface are necessary for this purpose. However, the etching rate during dry etching is not constant in time. As an alternative, several wet chemical etching procedures have been investigated.
A frequently used wet chemical solution exposed GaAs layers to a mixture of an acid (usually sulfuric acid H2SO4, phosphoric acid H3PO4, or citric acid C6H8O7) and hydrogen peroxide (H2O2), along with purified water (H2O) to dilute the solution and thus reduce the etch rate. It is also possible to use hydrochloric acid (HCl) along with nitric acid (HNO3), as the oxidizer. Hydrogen peroxide or nitric acid have the task of oxidizing the semiconductor surface. The oxides are etched away by the acids. Depending on the concentration of the solution, a desired etching rate can be achieved. However, the etching rate changes very sensitively with the concentration and the temperature of the solution, with its pH value (potential or power of hydrogen) and with its movements. Typically the etch rates are time dependent and nonuniform across the surface, unless a large volume of the solution is continuously mixed, to achieve a constant value at the etched surface [5]. Wet etching is also negatively impacted by the surface features of the VCSEL, such as the top metal ring contact and by any surface defects, thus generally resulting in unsatisfactory results being not well suited for high volume manufacturing.
A particularly precise control of the etch depth is claimed by the use of digital etching [6]. Here, oxidation and the etching of the oxides are separated from each other by two alternating repeatable steps. The surface is oxidized with hydrogen peroxide without the presence of an acid. This process is limited by diffusion and results in a precise oxide thickness on the GaAs surface for VCSELs using GaAs-based top mirrors, or any other surface for other types of VCSELs. For a very broad time window (for example between 5 s and 120 s) of exposure to the hydrogen peroxide an oxidation depth limited to about 15 nm is achieved [6]. The oxide is then removed by the acid in the absence of the hydrogen peroxide. The acid removes the oxidized GaAs leaving a fresh GaAs surface which may then be again oxidized. The treatment with the acid must be strictly separated from the treatment with the hydrogen peroxide, such that the removal of oxidized layers of GaAs proceeds in steps. To ensure this, the structures are rinsed with ultrapure deionized water after each step and dried using a wafer spinner. The use of HCl can lead to problems with the metal contacts. The use of C6H8O7 is believed to be more gentle compared to the other acids. The temperature of H2SO4:H2O2:H2O must be monitored since the initial mixture results in an exothermic reaction (temperature rise). This digital etching method has been reported to result in reliable etching depths. However, any surface roughness, surface defect, or geometric proximity effect renders this technique unreliable. The exposed GaAs mirror surface is strongly attacked, irrespective of the acid or etching solution used and the method of application. FIG. 1 as an example, shows this very clearly.
More specifically, FIG. 1 shows scanning electron micrographs of the GaAs mirror surface of completely processed VCSELs after digital wet etching using the technique described in [6]. Within the top metal ring contact, the GaAs is pitted, the etching is nonuniform, and surface defects lead to serious cracking. Additionally, residual photoresist results in obvious unwanted features near the inner edges of the metal rings.
The digital technique is unstable and not suitable for volume production.
Altering the cavity photon lifetime by wet or dry etching is thus at least complex and/or uncontrollable, typically resulting in unintended destruction of the VCSEL surface leading to reliability problems.
The invention as described hereinafter proposes a simple and nondestructive method to address the problems discussed above.